1. Field of the Invention:
The present invention generally relates to a star network optical transmission system, and more particularly to a star network optical transmission system which employes an optical transmission apparatus comprising a star coupler as a center node and a laser diode (LD) as a light source to effect optical fiber transmission and further particularly to a star network optical transmission system in which optical signal is branched and transmitted via a star coupler.
2. Prior Art
FIG. 5 is a diagram illustrating the configuration of the prior art star network optical transmission system as disclosed in "Toshiba Review", vol. 40, No. 7 (1985), p. 627-629 ("Optical Star Network Using Wavelength-Division Multiplexing"). In FIG. 5, shown at 1, 2, 3 and 4 are a transmission apparatus, a laser diode (LD), an up optical fiber comprising a multimode optical fiber and a star coupler for multimode optical fiber, respectively. Shown at 5, 10 and 11 are optical fibers at the input portion in the star coupler. Shown at 6 is a mixer portion in the star coupler 4. Shown at 7, 8 and 9 are optical fibers at the output portion in the star coupler 4. Shown at 12, 13 and 14 are a down optical fiber comprising a multimode optical fiber, a reception apparatus and a light receiving element, respectively.
FIG. 6 is a diagram illustrating the operation of the star network optical transmission system of FIG. 5. In FIG. 6, shown at 3a, 5a, 5b and 7a are outer diameter of the core of the up optical fiber 3, outer diameter of the core of one end of the optical fiber 5 at the input portion, outer diameter of the core of the other end of the optical fiber 5 at the input portion and outer diameter of the core of the optical fiber 7 at the output portion, respectively. Like numerals are used where the components are the same as those of FIG. 5. The detailed description of those components is omitted.
The operation of the prior art star network optical transmission system will be described hereinafter. The transmission apparatus 1 converts transmission signal to optical signal of a wavelength .lambda. through the laser diode 2, and then outputs it to the up optical fiber 3 comprising a multimode optical fiber exhibiting a multimode transmission characteristic to optical signal of a wavelength .lambda.. The laser diode 2 is a commonly used Fabry-Perot type laser diode and is adapted to oscillate in a multivertical mode during modulation. The up optical fiber 3 is connected to the optical fiber 5 at the input portion in the star coupler 4 for multimode optical fiber. The optical signal inputted to the optical fiber 5 at the input portion is then inputted to the mixer portion 6 where it is propagated so that beams are equally dispersed and coupled to the plurality of optical fibers 7, 8 and 9 at the output portion in principle. If there are further provided optical fibers 10 and 11 at the input portion, each fiber output is equally dispersed and coupled to the optical fibers 7, 8 and 9 at the output portion. The optical fibers 5, 10 and 11 at the input portion and the optical fibers 7, 8 and 9 at the output portion exhibit a multimode transmission characteristic to a wavelength of .lambda.. The optical fiber 7 at the output portion in the star coupler 4 is connected to the down optical fiber 12 comprising a multimode transmission characteristic to optical signal of a wavelength .lambda.. The optical signal transmitted to the down optical fiber 12 is then optoelectrically converted by the light receiving element 14 in the reception apparatus 13. Thus, the signal from the transmission apparatus 1 is received.
The above described laser diode 2 is a light source having a strong coherence. When the optical signal outputted from the laser diode 2 is coupled and transmitted to the up optical fiber 3, the power strength distribution developed in the core outer diameter 3a is in a speckle pattern of large particles because of coherence as shown in FIG. 6. If there is an axis shift or the like between the core outer diameter 3a of the up optical fiber 3 and the core outer diameter 5a of one end of the optical fiber 5 at the input portion in the star coupler 4, only a part of the speckle pattern of the former fiber is coupled to the speckle pattern of the latter fiber. Also, in the end surface of the connection between the optical fiber 5 at the input portion and the mixer portion 6, a speckle pattern is developed in the core outer diameter 5b of the other end of the optical fiber 5 at the input portion. In the mixer portion 6 of the star coupler 4, optical signal outputted from the optical fiber 5 at the input portion is propagated while undergoing the multiple reflection between the walls of the mixer portion 6 so that a uniform optical power distribution is developed on the end surface of the output. However, a speckle pattern is still left. A stable optical power is coupled to the core outer diameter 7a of the optical fiber 7 at the output portion so far as such a speckle pattern is stable.
Configured as described above, the prior art star network optical transmission system is disadvantageous in that the vibration of the up optical fiber 3 or the temperature characteristics of the laser diode 2 causes a fluctuation in the speckle pattern developed on the end surface of the connection between the up optical fiber 3 and the optical fiber 5 at the input portion which can cause a deviation in the rate of coupling to the optical fibers 7, 8 and 9 at the output end surface in the mixer portion 6, resulting in the generation of so-called modal noise which deteriorates S/N ratio of transmission signal.
Further, if there is an axis shift in the end surface of the connection between the optical fiber 5 at the input portion and the optical fiber 3 in the star coupler 4, such a deterioration in S/N ratio becomes remarkable. Furthermore, since the laser diode oscillates in a multivertical mode, a phenomenon called mode distribution characteristic causes a fluctuation in the oscillation spectrum for every pulse of transmission data during data transmission at a high rate of 100 Mb/s or more, worsening the effect of modal noise.
The present invention has been attained in order to overcome these programs. It is an object of the present invention to provide a star network optical transmission system which reduces the fluctuation in the speckle pattern developed on the output end surface of a mixer portion in a star coupler caused by the vibration of an optical fiber or the temperature characteristics or mode distribution characteristics of a laser diode to obtain a stable coupling characteristic, enabling signal transmission with a high S/N ratio.
Further, FIG. 12 shows another example of the prior art star network optical transmission system as described in D. W. Faulkner, "Broadcast Television via Passive Optical Networks", 13th ECOC Technical Digest vol. 1, p. 283 (1987). Shown at 122, 122, 123 and 124 in FIG. 12 are an optical transmission apparatus, a one-input 2N (N is an integer) output star coupler (hereinafter simply referred to as "[1.times.2N] star coupler"), and a 1st and 2Nth optical reception apparatus, respectively. Shown at 125, 126a and 126b are optical fibers. Shown at 127 and 128 are an input optical fiber and a mixer portion. Shown at 129a and 129b are output optical fibers. FIG. 13 is a diagram illustrating the operation of the star coupler 122. Optical signal inputted from the input optical fiber 127 is diffused at the mixer portion 128, and then distributed into the output optical fibers 129a and 129b.
The operation of the conventional star network optical transmission system will be described with reference to FIG. 12. In this optical transmission system, optical signal transmitted by the optical transmission apparatus 121 is branched by the star coupler 122 to 2N branches, and then transmitted to the optical reception apparatus 123 and the 2Nth optical reception apparatus 124.
Optical signal transmitted by the optical transmission apparatus 121 is transmitted to the star coupler 122 via the optical fiber 125. In the star coupler 122, the transmitted optical signal is then propagated by the input optical fiber 127 to the mixer portion 128 where it is diffused and then equally distributed to 2N output optical fibers 129a to 129b. The optical signals thus equally distributed are transmitted to the 1st to 2Nth optical reception apparatus 123 to 124 via the optical fibers 126a to 126b connected to the output optical fibers 129a to 129b, respectively.
Configured as described above, the conventional star network optical transmission system requires the use of a star coupler having the same number of output ports as that of the branches or a plurality of star couplers having a less number of output ports to form a multibranch system. In this configuration, as the number of branches increases, the number of branch systems increases, making the system larger in size and expensive to implement. Furthermore, in order to manufacture a [1.times.N] star coupler having a plurality of output ports, a [2N.times.2N] star coupler having a plurality of input ports and output ports is actually manufactured in the manufacturing process. These input ports of the star coupler thus manufactured are used singly. Thus, such a star coupler exhibits a low cost performance.
The present invention has been attained in order to overcome these problems. It is an object of the present invention to provide a star network optical transmission system which uses an [N+1].times.N+1] star coupler to enable 2N branching of optical signal.